1. Field of the Invention
The present invention relates to a system for transmitting encoded signals representing image data, and more particularly, to such a system used in a TV conference system and TV telephone system using ATM (asynchronous transfer mode) network by which a multiple view image data are produced from a plurality of image data transmitted in the ATM network.
2. Description of the Background Art
A TV conference system is a system for enabling a conference among distanced participants by providing audio and visual communications among the participants through a communication network connecting a plurality of display terminals, so as to simulate a conference in a conference room. Currently, a progress has been made in realizing such a TV conference system by utilizing a wide range ISDN ATM network.
In this TV conference system, it is usually necessary to provide each display terminal of each participant with views of the other participants on a single display, in a form of a multiple view image produced by contracting each view of each participant and arranging such contracted views of the other participants on a single display.
For this reason, a data transmission in such a TV conference system takes a form of encoding the image data at each user terminal and transmitting the encoded image data through the ATM network; producing the encoded multiple view image data at an image bridge circuit in the ATM network from a plurality of encoded image data transmitted in the ATM network by applying an image contraction processing; and decoding the encoded multiple view image data transmitted through the ATM network at each user terminal.
Conventionally, such an image bridge circuit in the ATM network operates by first decoding the encoded image data received from the user terminal through the ATM network, then producing the multiple view image data by applying an image contraction processing to each decoded image data and arranging such contracted image data, and encoding the obtained multiple view image data and transmitting the encoded multiple view image data out to the ATM network. Such a conventional image bridge circuit therefore required a plurality of decoders for decoding the plurality of image data from the plurality of user terminals connected to it, and one or more encoders for encoding the image obtained by it, so that the size of the image bridge circuit itself inevitably becomes quite large, which consequently also contributed to the undesirable increase of the size of the system as a whole.
Moreover, the processing at the conventional image bridge circuit with the configuration described above can be quite time consuming, especially when the number of users becomes large, to a point of causing a practically problematic and undesirable delay in the data transmission, and yet this problem of the delay in the data transmission due to the time consuming processing at the image bridge circuit can be alleviated only by severely limiting a number of user terminals involved in the TV conference system to an unreasonably small number.
More specifically, the image data have been expanded or contracted conventionally by row by row (or column by column) filtering and sub-sampling for the case of contraction, and row by row (or column by column) filtering and interpolation for the case of expansion, which will now be descried in terms of diagrams shown in FIGS. 1(A) to 1(E).
FIG. 1(A) is a diagram showing picture element values for one row of the picture elements in an image in a form of a wave. By Fourier transforming this diagram of FIG. 1(A), a frequency characteristic shown in FIG. 1(B) can be obtained. In FIG. 1(B), fm indicates a sampling frequency.
In a case of contracting, in order to reduce a number of picture elements, the sampling frequency have to be lowered from fm to a lower frequency fx. Here, the high frequency components in the original signals are suppressed by using an appropriate filtering such as shown in FIG. 1(C) in order to avoid the turning back of the frequency components between (1/2)fx and (1/2)fm to obtain the frequency characteristic for the contracted image shown in FIG. 1(D).
On the other hand, in a case of expanding, the sampling frequency is raised from fm to a higher frequency fy, and by using an appropriate filtering the frequency characteristic for the expanded image shown in FIG. 1(E) can be obtained.
Now, for encoding the image data, there is a method known as an orthogonal transform encoding in which the image is divided into a plurality of (usually rectangular) blocks and the orthogonal transform is carried out for each one of these blocks, and then the transform coefficients of the orthogonal transform are quantized and encoded.
In a case of expanding or contracting the image data encoded by such an encoding method, it has conventionally been necessary to reproduce the entire original image by decoding, before the application the sub-sampling or interpolation operation for the sake of contraction or expansion.
For this reason, it has conventionally necessary to separately provide a circuit for reproducing the entire original image, a frame memory capable of memorizing the entire original image, and a circuit for applying an operation for the expansion and contraction.
Moreover, such a conventional method of expanding or contracting the encoded image data is quite time consuming as it involves the reproduction of the entire original image.